The applicant proposed in Japanese Patent Publication (A) No. 2007-303423 a spark ignition type internal combustion engine, comprising a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to change a closing timing of the intake valve, wherein the mechanism compression ratio is raised at the time of engine low load operation compared with engine high load operation, to make an expansion ratio 20 or more.
In such a spark ignition type internal combustion engine, at the time of engine low load operation, the mechanical compression ratio (expansion ratio) is made 20 or more and the closing timing of the intake valve is made a timing away from intake bottom dead center so as to maintain the actual compression ratio relatively low compared with mechanical compression ratio, to suppress the occurrence of knocking due to the actual compression ratio becoming higher, and while doing so realize an extremely high heat efficiency.
In this regard, when using a variable compression ratio mechanism such as described in Japanese Patent Publication (A) No. 2007-303423, the higher the mechanical compression ratio, the smaller the volume of a combustion chamber when the piston is at top dead center, therefore the larger the surface-volume ratio (ratio of the surface area and volume of a combustion chamber, hereinafter referred to as “the S/V ratio”). If the S/V ratio becomes larger in this way, the quench region (region near the wall surfaces of a combustion chamber etc. which flame cannot reach) becomes relatively larger. The HC in the air-fuel mixture which was included in this quench region will not burn since even if the air-fuel mixture in the combustion chamber burns, the flame will not reach it. On the other hand, the HC in the air-fuel mixture which was included in this quench region is exposed to a high temperature along with combustion of the air-fuel mixture, so part is converted to hydrogen (H2). That is, if using a variable compression ratio mechanism to raise the mechanical compression ratio, the S/V ratio will increase and therefore the H2 in the exhaust gas will increase.
On the other hand, in many internal combustion engines, for the purpose of increasing the efficiency of combustion and improving exhaust emissions, the air-fuel ratio of an air-fuel mixture which is fed into a combustion chamber is maintained at a target air-fuel ratio (for example, stoichiometric air-fuel ratio) by using an oxygen sensor or an air-fuel ratio sensor. However, an oxygen sensor and air-fuel ratio sensor are highly sensitive to H2. If the amount of H2 generated increases, the output value tends to deviate to the rich side.
In particular, in the above-mentioned type of spark ignition type internal combustion engine where the mechanical compression ratio becomes 20 or more, the S/V ratio becomes extremely large and along with this the amount of H2 which is exhausted from a combustion chamber also becomes large. For this reason, the output value of the oxygen sensor or air-fuel ratio sensor greatly deviate to the rich side to an extent which cannot be ignored, and the oxygen concentration etc. in the exhaust gas can no longer be accurately detected. As a result, the air-fuel ratio can no longer be suitably controlled and deterioration of the combustion efficiency or deterioration of the exhaust emission is invited in some cases.